In conventional AC-motors, average rotor current measurements are oftentimes adopted for estimating a rotor temperature. Examples of such motors are universal motors for driving various kinds of power tools and other equipment, as well as motors specifically adapted for a certain application, such as for a power cutters or for drilling machines, and particularly for driving a drill bit of a core drill via a water cooled gearbox. In the latter case the motor can be slidably mounted on a drill stand, which is anchored to the ground/floor and a core drill is arranged on an output shaft of the gearbox to be rotatably driven by the motor. Sliding the motor slowly down the drill stand feeds the drill bit deeper into the ground/floor/structure to be drilled.
The rotor is usually the most temperature exposed part of an AC-motor having an outer stator and an inner/central rotor fitted with rotor windings. At high load, the rotor windings warm up the rotor significantly and, depending on design, size and power dimensioning, the rotor may under certain operational conditions run the risk of overheating. As an example the rotor might withstand temperatures up to 180° C., but would be overheated and break down at higher temperatures. Since it is difficult and costly to measure the actual rotor temperature, it can instead be estimated. For example in Husqvarna motors provided with overheat protection, Elgard™, the current through the rotor is measured and if the current reaches a certain threshold value, the motor is switched into a pulsation mode, in which the current fed to the rotor is pulsating. A typical pulsation cycle time is 0.5 seconds. If the load of the motor is not reduced during the pulsation, the current is completely switched off if the motor load is not reduced within a predetermined time period. Since the rotor temperature depends not only on rotor current but also on other operational conditions, the full potential of the motor (and rotor) is not always available. To provide a motor in a certain power range with excellent reliability, it is therefore usually required to over-dimension the motor. However, over-dimensioning makes the motor more unwieldy, heavier, and more expensive than otherwise would be necessary.